Despite much current interest in the evolution of thermoregulation, three fundamental issues are virtually unexplored: individual variation, maternal effects, and inheritance of thermoregulation. All three issues will be investigated in a single, natural population of live-bearing garter snakes (Thamnophis) using a uniquely integrated series of field and laboratory experiments. To document individual variation in maternal thermoregulation, body temperatures will be continuously monitored via radiotelemetry throughout pregnancy in both free-ranging females and in captive females exposed to thermal gradients. The field measurements will reveal how much body temperature variation actually occurs in nature, and the laboratory measurements will provide greater discrimination of intrinsic differences in thermoregulation. To determine the potential range of effects of maternal thermoregulation, another group of captive snakes will be exposed to a variety of constant and cycling temperature treatments during pregnancy. The direct consequences of both natural and artificially induced variation in maternal thermoregulation will be evaluated by correlating thermoregulatory differences with the lengths of gestation periods. The indirect consequences of variation in maternal thermoregulation will be investigated by scoring meristic (i.e., countable) characters (e.g., number of vertebrae) in parents and offspring. Meristic traits in garter snakes are ideal for the project because they are known to be sensitive to developmental temperature, to be heritable, and to be affected by natural selection. Genetic variances (heritabilities) of thermoregulation and meristic traits, and the genetic covariance between these traits, will be estimated by measuring resemblance between parents and offspring. Recent, quantitative genetic models of evolution will provide the conceptual framework for synthesizing the results. These models highlight the role of heritable variation in maternal thermoregulation and also reveal the crucial importance of a genetic covariance that could have dramatic impact on the evolution of both thermoregulation and temperature sensitive traits. Counter-intuitive evolutionary responses can result if the sign of the genetic covariance is negative.